QR codes on receipts: the post-purchase moment, printed at 203 dpi

Receipt QR codes work because the moment is post-purchase: a review request, a digital copy, a warranty registration or a fiscal-authority code. On a 203 dpi thermal printer, size the code so each module is a whole number of printer dots, and keep it clear of the tear line.

Why the receipt slot is valuable

A receipt reaches exactly one audience: someone who has just bought from you, holding a piece of your paper voluntarily. That timing is what makes the slot worth designing rather than wasting on a homepage link. The candidates, roughly in order of measurable value:

A review request: the highest-converting moment for Google review codes is minutes after a good experience, and the review tool deep-links straight to the review form.
Feedback, a private channel for the unhappy customer who would otherwise become a public one-star review.
Digital receipt retrieval: scan to fetch a PDF copy for expenses; the paper can fade (direct thermal print fades badly), the digital copy does not.
Warranty registration or care instructions, for merchants selling durable goods.
A tip code where tipping happens after payment.

Pick one. A receipt is ~80 mm wide; two or three competing codes on it split attention and scans, the multiple-codes problem in miniature. Decide what the post-purchase moment is for in your business and give the slot to that.

Fiscal receipt codes: the mandated kind

In a growing number of countries the receipt QR is not marketing but law: tax authorities mandate a QR on fiscal receipts carrying a verification payload (receipt identifier, amounts, a signature or lookup code), so a buyer (or inspector) can confirm the receipt was reported to the tax system. Several countries pair this with receipt lotteries that give consumers a reason to collect and check receipts, which is a genuinely clever compliance mechanism: it turns every customer into an auditor.

If you operate somewhere with such a mandate, that code's content, format and placement are specified by the authority and generated by your certified till system. It is not yours to design, and it occupies its slot by right. Your marketing code, if any, goes elsewhere on the paper, clearly separated so nobody scans the wrong one.

Surviving the thermal printer

Receipt printers are the hostile end of QR printing. Most run 203 dpi (8 dots per millimetre), on paper with mediocre contrast, and thermal-specific rules apply:

Whole-dot modules. Render so each module is an integer number of printer dots, e.g. 4 dots = 0.5 mm modules. Fractional-dot scaling makes alternating module sizes and kills scans.
Keep the payload short so the module count stays low and modules stay big: a short URL beats a long parameterised one here more than anywhere (density costs scans).
Quiet zone ≥ 4 modules, which on a crowded 80 mm receipt means resisting the urge to cram text against the code.
Placement: end of the receipt, clear of the tear line. A code that straddles the tear gets decapitated by design. Bottom placement also means the customer sees it as the receipt is handed over, which is when your one-line prompt ("Scan to rate us, 10 seconds") does its work.
Test on the actual printer, not a laser proof, then verify the printed result scans with a real phone under shop lighting.

FAQ

What should a receipt QR code link to?

One thing, chosen deliberately: a review request, private feedback, a digital-receipt download, warranty registration or a tip code. The post-purchase moment is valuable, splitting it across several codes wastes it.

Why do some countries require QR codes on receipts?

Tax authorities mandate verification QRs on fiscal receipts so buyers and inspectors can confirm the sale was reported. Some pair it with receipt lotteries to give consumers an incentive to collect and check receipts.

How big should a QR code be on a thermal receipt?

Size it so each module is a whole number of printer dots (at 203 dpi, four dots per module gives 0.5 mm modules) keep the payload short, preserve a four-module quiet zone, and place it clear of the tear line.

Why does my receipt QR code stop scanning after a few weeks?

Direct thermal paper fades with heat, light and time, taking contrast below scannable levels. That argues for putting durable content (warranty, manuals) behind a code the customer scans promptly, not months later.